Appropriate nesting – Failed

Appropriate nesting – Failed is the result of the only rule in the Headings category of Adobe Acrobat Pro's Accessibility Checker. It means the document's heading tags (H1 to H6, the labels in its hidden structure that mark headings for assistive technology) aren't nested properly. A common cause is a skipped level, such as an H3 straight after an H1. Headings are how many screen reader users skim a document, so their levels need to form a clean outline.

What this means

Heading levels should work like an outline:

• H1 for the document's title or main heading.
• H2 for the main sections.
• H3 for subsections inside an H2, and so on.

Going down, levels go one step at a time: H2 to H3, never H2 to H4. Going back up can jump: an H4 followed by a new H2 section is fine.

PDF/UA, the ISO standard for accessible PDFs, is stricter than WCAG here. Adobe's own help notes that heading order is advisory rather than required under WCAG. PDF/UA-1 does require it: if a document uses numbered headings, the first must be H1, and levels can't be skipped on the way down. PDF/UA also says not to mix the generic H tag with numbered H1 to H6 tags. So a file can pass Acrobat's rule and still fail PAC, the free PDF Accessibility Checker.

Common causes:

• Headings chosen for their look in the source, such as picking Heading 3 because it's the right size.
• No H1, because the document title is just large text.
• Documents merged from several files, each with its own outline.
• Autotagging, which guesses heading levels from font size.

One trap works the other way: a document with no heading tags at all can pass this rule, because there's nothing to nest. It's still hard to navigate, and Includoc flags multi-page documents with no headings separately.

Why it matters

Screen reader users often bring up a list of headings, or jump from heading to heading, to find what they need. Skipped levels suggest missing sections and make the outline hard to follow. Missing headings are worse: people have to listen from the start to find anything. This affects WCAG 2.1 success criteria 1.3.1 Info and Relationships and 2.4.6 Headings and Labels, and it matters for people with cognitive disabilities, who rely on a clear structure.

How to fix it in Acrobat Pro

Fix it in the source file

Heading levels are much easier to get right in the source:

• Word: apply Heading 1, Heading 2 and so on from the Styles gallery. To change how headings look, modify the style instead of choosing a different level. View > Navigation Pane shows the outline as you work, and Word's Accessibility Checker gives a tip when a document doesn't use heading styles.
• InDesign: map each heading paragraph style to H1 to H6 in its Export Tagging settings.
• Everywhere: use one H1 for the title, and don't skip levels on the way down.

Export with tags turned on, and the PDF's headings will follow your outline. Bookmarks built from headings will follow it too.

Fix it automatically with Includoc

Includoc fixes the mechanical part automatically. We renumber headings so levels go down one step at a time, and convert generic H tags to numbered H1 to H6 tags.

Choosing the right levels takes judgment, so it's assisted. Our AI detects headings from font size and weight, including text that looks like a heading but isn't tagged as one, and proposes a level for each. You confirm the outline in the headings panel on the review screen before you download the file.

Standards

Related errors

• Bookmarks – Failed: bookmarks are usually built from headings, so fix the headings first.
• List items and Lbl and LBody – Failed: another part of the document's structure that Acrobat checks.

For the full workflow, read How to make a PDF accessible, or check a PDF free to see your document's heading outline.